The present invention relates generally to electronic ballasts for powering discharge lamps, and light fixtures making use of the same.
Discharge lamp ballasts for powering HID lamps utilizing arc discharge in high-voltage metal vapor are conventionally known in the art. With reference to FIG. 21, such a discharge lamp ballast is provided with a DC power supply 2 which converts AC power input from an AC power source 1 to DC power with a predetermined voltage, an inverter 3 which converts the DC power output from the DC power supply 2 to AC power and provides the AC power to a discharge lamp DL, and a control circuit 4 which controls the inverter 3.
The DC power supply 2 as shown in FIG. 21 is provided with a diode bridge DB connected to the AC power source 1 via a proper fuse or filter, a DC output terminal on a low voltage side thereof connected to ground, a series circuit of an inductor L3 and a switching element Q5 connected across DC output terminals of the diode bridge DB, and a series circuit of a diode D5 and an output capacitor C5 connected in parallel with the switching element Q5. The DC power supply 2 as shown provides a voltage across the output capacitor C5 as an output voltage. That is, the DC power supply 2 includes a well-known boost converter (a step-up chopper circuit, or power factor correction (PFC) circuit) connected across output terminals of the diode bridge DB. Further, the DC power supply 2 is provided with a power supply drive circuit 21 for ON/OFF-driving the switching element Q5 periodically at such an on-duty as to keep the voltage across the output capacitor C5 (namely, the output voltage of the DC power supply 2) constant.
The inverter 3 as shown in FIG. 21 includes two sets of series circuits in a full-bridge configuration, each set connected across output terminals of the DC power supply 2 and including two of the switching elements Q1 to Q4. Each of the switching elements Q1 to Q4 in the inverter 3 is, for example, a MOSFET including a parasitic diode (a body diode), and is connected such that a forward direction of the body diode is a direction opposite to the output of the DC power supply 2. Two switching elements Q2 and Q4 on a low voltage side of the switching elements Q1 to Q4 are each connected to an output terminal on the low voltage side of the DC power supply 2 via a sense resistor R2.
The discharge lamp DL in the example shown may be an HID lamp (“high-pressure discharge lamp”), one end of the discharge lamp DL being connected to a node between switching elements Q3 and Q4 of one of the series circuits (the switching element Q3 on the high voltage side of these switching elements Q3 and Q4 is hereinafter called “third switching element Q3”, while the switching element Q4 on the low voltage side thereof is hereinafter called “fourth switching element Q4”) via a first inductor L1. The other end of the lamp DL is connected to a node between switching elements Q1 and Q2 of the other of the series circuits (the switching element Q1 on the high voltage side of these switching elements Q1 and Q2 is hereinafter called “first switching element Q1”, while the switching element Q2 on the low voltage side thereof is hereinafter called “second switching element Q2”) via a second inductor L2.
A first capacitor C1 is connected in parallel with the series circuit of the second inductor L2 and the discharge lamp DL for ripple reduction. Furthermore, a node between switching elements Q2 and Q4 on the low voltage side and the sense resistor R2 is connected to ground via a series circuit of two diodes D1 and D2, the cathode of each of the diodes being directed toward the ground, and a resistor. The diodes D1 and D2 cause current flowing in a resonant circuit configured by a second capacitor C2 and the second inductor L2 (i.e., resonant current) to bypass the sense resistor R2 such that the resonant current does not flow in the sense resistor R2. Further, the second inductor L2 as shown is configured as an auto-transformer with a tap connected to a node between the two diodes D1 and D2 via a series circuit of the second capacitor C2 and the resistor R1.
The control circuit 4 is provided with switch drive circuits 41 and 42 which drive ON/OFF the respective switching elements Q1 to Q4 in the inverter 3, and an integrated circuit 40 or the equivalent which controls the drive circuits 41 and 42. Because such a control circuit 4 can be realized utilizing a number of well-known techniques, detailed illustration and explanation are omitted.
The integrated circuit 40 is connected to a node between the sense resistor R2 and the switching elements Q2 and Q4, and a node between the second inductor L2 and the switching elements Q1 and Q2. That is, the integrated circuit 40 detects output current (hereinafter, called “lamp current”) from the inverter 3 to the discharge lamp DL based upon a voltage across the sense resistor R2 and detects an output voltage (hereinafter, called “lamp voltage”) from the inverter 3 to the discharge lamp DL based upon a potential of a node between the second inductor L2 and the switching elements Q1 and Q2 with respect to ground. Further, the first inductor L1 is provided with a secondary winding having a tap connected to the ground, and both ends of the secondary winding are connected to the integrated circuit 40 via diodes D3 and D4, respectively.
Operation of the above-mentioned discharge lamp ballast will be explained below. When power is turned on, the control circuit 4 performs a starting operation which causes the inverter 3 to output a high voltage as required to initiate discharge in the discharge lamp DL. Specifically, the respective switching elements Q1 to Q4 in the inverter 3 are driven ON/OFF periodically such that switching elements Q1 to Q4 which are positioned diagonally to each other in the full-bridge configuration are simultaneously turned on, and those connected serially to each other are alternately turned on. The frequency for ON/OFF-driving is set to a resonant frequency (for example, 120 kHz which is a secondary resonant frequency of one third of a reference resonant frequency 360 kHz) of the resonant circuit including the second inductor L2 and the second capacitor C2. During the starting operation, a resonant voltage generated at a node between the second inductor L2 and the second capacitor C2 is boosted at the second inductor L2 serving as the auto-transformer to be output to the discharge lamp DL, and thereby discharge lamp DL lamp ignites. When the discharge lamp DL ignites, current starts to flow in the discharge lamp DL, wherein the control circuit 4 detects lighting of the discharge lamp DL based upon current induction by the secondary winding in the first inductor L1 to flow in the discharge lamp DL via the diodes D3 and D4, and operation of the discharge lamp DL proceeds next to an electrode heating operation.
The electrode heating operation P2 will be explained with reference to FIG. 22. The horizontal axis represents time, with each of graphs Q1 to Q4 indicating an ON-period of the associated switching element. The same holds true for various figures further described below. In the electrode heating operation P2, as shown in FIG. 22, the control circuit 4 first turns on respective switching elements (for example, the first switching element Q1 and the fourth switching element Q4) in one set of switching elements Q1 to Q4 positioned diagonally to each other, and turns off the respective switching elements (for example, the second switching element Q2 and the third switching element Q3) in the other set, respectively. Thereby, current (hereinafter, called “circuit current”) flowing in the first inductor L1 gradually increases, so that energy is accumulated in the respective inductors L1 and L2, respectively.
When the circuit current has reached a predetermined value, the control circuit 4 turns off one switching element (for example, the fourth switching element Q4) in the set which has been turned on. After a predetermined period of time has elapsed, the control circuit 4 turns off the other switching element (for example, the switching element Q1). During a period where at least three of the switching elements Q1 to Q4 have been turned off, the circuit current flows in a gradually decreasing manner due to energy discharge from the respective inductors L1 and L2 through a loop including the body diodes of the switching elements Q1 and Q4 which were previously in an ON state and the output capacitor C5 in the DC power supply 2. When the circuit current has reached zero, the control circuit 4 turns on the respective switching elements Q1 and Q4 in the one set, and it repeats a similar operation for a predetermined number of times. Thereafter, as shown by the right half of the chart in FIG. 22, the switch sets are reversed with regards to the ON/OFF control operation.
In the above example, the first switching element Q1 and the fourth switching element Q4 are maintained in off states, respectively, while the second switching element Q2 and the third switching element Q3 are now controlled ON/OFF as described above. Thereby, the polarity of the circuit current is inverted and again as previously described a similar ON/OFF-control is repeated for a predetermined number of times. This operation of switch set reversal and polarity inversion with regards to the circuit current may again be repeated as needed or as predetermined thereafter. This polarity inversion of the circuit current may be performed at a frequency of 100 Hz to 200 Hz.
When the lamp voltage has reached a predetermined voltage during the electrode heating operation P2, the control circuit 4 makes a transition to a stable (i.e., “steady-state” as further referred to herein) lighting operation P3 for maintaining lighting of the discharge lamp DL. In the steady-state lighting operation P3, as shown in FIG. 23, the control circuit 4 first turns on respective switching elements (for example, the first switching element Q1 and the fourth switching element Q4) in one set of the switching elements Q1 to Q4 positioned diagonally to each other and turns off the respective switching elements (for example, the second switching element Q2 and the third switching element Q3) in the other set, respectively. Thereby, the circuit current gradually increases, so that energy is accumulated in the respective inductors L1 and L2, respectively. When the circuit current has reached a predetermined value, the control circuit 4 turns off one switching element (for example, the fourth switching element Q4) in the set which has been turned on. Thereby, the circuit current flows in a gradually decreasing manner due to energy discharge from the respective inductors L1 and L2 through a loop including the body diodes of the switching elements Q1 and Q4 which were previously in an ON state and the output capacitor C5 in the DC power supply 2. When the circuit current has reached zero, the control circuit 4 turns on the one switching element (the fourth switching element Q4 in the above example) and it repeats a similar operation for a predetermined number of times. Thereafter, as shown in the right half of the chart in FIG. 23, the switch sets are reversed with regards to the ON/OFF control operation.
In the above example, the first switching element Q1 and the fourth switching element Q4 are maintained in off states, respectively, while the second switching element Q2 and the third switching element Q3 are now controlled ON/OFF as described above. Thereby, the polarity of the circuit current is inverted and again as previously described a similar ON/OFF-control is repeated for a predetermined number of times. This operation of switch set reversal and polarity inversion with regards to the circuit current may again be repeated as needed or as predetermined thereafter. This polarity inversion of the circuit current may be performed at a frequency of 100 Hz to 200 Hz.
In the abovementioned discharge lamp ballast, the frequency of the output to the discharge lamp DL during the starting operation is set to be very high as compared with the frequency thereof during the electrode heating operation or the steady-state lighting operation performed thereafter. Accordingly, even if the discharge lamp DL during the starting operation starts to ignite, current flows in the first capacitor C1 connected in parallel with the discharge lamp DL, and extinguishing of the discharge lamp DL easily occurs due to the lack of lamp current required for maintaining discharge in the discharge lamp DL.